Cohort Type: At present, you can only use Acquisition date (First Visitor Session to Website)
Hope, in future, google will relish more cohort type

Cohort Size: It is nothing but the interval of cohort time window (Day, Week, Month)

Metric: Actual data point that needs to be Analyzyed (Like retention, Pageviews, etc)

Date Range: Time Window

Pls. Note: Maximum you can see 12 columns. For example, If you choose the date range Last 30 days & cohort Size is "By day", will only see 12 Interval or 12 days. If you for the month, move to Week Interval

How to make Cohort Analysis for better insight?
Combine cohort Analysis with Advanced Segmentation for better insight

Lets take few behavior insight can be emerged in Cohort Analysis

What should be the retention behavior on different device category (Desktop, Mobile,Tablet)?